Electrical enclosures are used to house and protect electronics equipment from potentially harmful environments such as high humidity or rain, condensation, solar heat loads, dust and debris, temperature extremes and damaging corrosion. In some cases, the cabinets must be sealed to a standard, typically by National Electrical Manufacturers Association (NEMA). For example, a NEMA 12 cabinet is for indoor use and protects against drips, dust and falling dirt. A NEMA 4 cabinet is for indoor or outdoor use and protects against the same things as NEMA 12 as well as hose directed spray. The sealed nature of these cabinets requires heat transfer from the inside space to the ambient environment while maintaining the NEMA rated seal.
For most electronic components in an enclosure, the suggested maximum allowable air temperature within the enclosure is in the range of 130° F. to 160° F. One common guideline states that for every 18° F. the temperature of an electronic component is elevated beyond its recommended operating temperature, the lifetime of the electronic components is cut in half. In order to increase electrical component lifetime and reduce process downtime due to component failure or replacement, a cooling solution must be chosen. If an enclosure must be sealed from the ambient environment, then a method of transferring heat through some cooling device must be chosen based on the amount of heat that needs to be removed and the ambient conditions around the enclosure.
In cases where the ambient air temperature is suitably low, an air-to-air heat exchanger 10, as shown diagrammatically in FIG. 1, can be used to transfer heat from the air 12 inside of the cabinet 14 to the ambient air 16. An air-to-air heat exchanger 10 transfers heat from the hot air 12 inside the enclosure 14 to the cooler ambient air 16 while maintaining a NEMA seal that prevents external air and other contaminants from entering the enclosure. It is beneficial to use a heat exchanger instead of an air conditioner (AC) when possible because the energy required to operate the cooling system is much lower.
Two key factors influencing the amount of heat that can be transferred from air inside of an enclosure to ambient air are the surface area available for heat transfer and the thermal resistance through the cooling device. Increasing the surface area of a heat exchanger by adding an extended surface or fin structure can increase heat transfer through the heat exchanger.
Lowering the thermal resistance between the internal enclosure air and the external ambient air reduces the difference between the two air temperatures that is necessary to transfer a given amount of heat. This means that with a lower thermal resistance, a lower internal enclosure temperature can be maintained for a given heat load into the enclosure. Air-to-liquid coolers are used for enclosures with high heat loads. These systems transfer heat to a liquid supply that may be a cooling water supply or a chilled liquid loop. In either case there is a requirement for additional equipment or a water supply that may not always be available.
Thermoelectric coolers are commonly used to achieve sub-ambient temperatures within an enclosure, but they are limited in capacity and are very expensive relative to other types of cooling products.
Air conditioners are also used to achieve sub-ambient cooling and are the most common active enclosure cooling product. The drawback to compressor-based cooling systems is that they require greater energy input than air-to-air heat exchangers.
When outside air cannot be introduced to the interior of an enclosure but the maximum temperature of the ambient environment and maximum heat load in the enclosure are suitable, a heat exchanger can be used to transfer heat out of the enclosure while maintaining a seal between the ambient environment and the interior of the enclosure. Heat exchanger products that are currently in use include heat pipe heat exchangers with aluminum finned coils, folded-fin heat exchangers, and plate & fin heat exchangers.